CRUMB MIXTURE:

2 c. all-purpose flour
1 c. firmly packed brown sugar
1/2 c. butter, softened
1 c. pecan halves, not chopped

FILLING:

2/3 c. butter
1/2 c. firmly packed brown sugar
1/2 c. semi-sweet chocolate pieces

In bowl, combine all crumb mixture ingredients. Beat at low speed until particles are fine, 1 to 2 minutes. Press into ungreased 9x13 inch baking pan. Sprinkle pecans evenly over unbaked crumb mixture.

Prepare filling; pour evenly over pecans and crumb mixture. Bake at 350 degrees for 18 to 20 minutes or until entire filling is bubbly. Immediately sprinkle with the chocolate pieces. Allow pieces to melt slightly, 2 to 5 minutes. Slightly swirl pieces as they melt; leave some whole for a marble effect. DO NOT SPREAD PIECES. Cool completely; cut into bars.

FILLING: In heavy 1-quart saucepan, combine butter and brown sugar. Cook over medium heat, stirring constantly, until mixture boils all over the top, 4 to 5 minutes. Boil 1/2 to 1 minute, stirring constantly.